This invention is generally directed to amorphous silicon imaging members, and more specifically, the present invention is directed to photoresponsive layered imaging members, or devices comprised of hydrogenated amorphous silicon and barrier layers of hydrogenated amorphous silicon nitride containing dopants such as boron. In one embodiment of the present invention there is provided a layered photoresponsive imaging member comprised of a supporting substrate, a barrier layer of hydrogenated amorphous silicon nitride with dopants therein, a bulk photoconducting layer of hydrogenated amorphous silicon with dopants, and in contact therewith an overcoating layer of silicon nitride, preferably with an excess of silicon. Further, in a specific embodiment of the present invention there is provided a layered photoresponsive imaging member comprised of a supporting substrate, a barrier layer of hydrogenated amorphous silicon nitride with small amounts of boron therein, a bulk boron doped photoconducting layer of hydrogenated amorphous silicon, and in contact therewith an overcoating layer of silicon nitride with an excess of silicon. These imaging members can be incorporated into electrographic, and in particular xerographic imaging and printing systems, wherein the latent electrostatic images which are formed can be developed into images of high quality, and excellent resolution. Moreover, these members posses high charge acceptance values in excess of 50 volts per micron, and the members can be of a very desirable thickness of from, for example, about 60 microns or less. Also, the imaging members of the present invention exhibit desirable low dark decay properties when selected for xerographic imaging systems. In these systems, latent electrostatic images are formed on the devices involved, followed by developing the images with known developer compositions, subsequently transferring the images to a suitable substrate, and optionally permanently affixing the image thereto. In addition, the photoresponsive imaging members of the present invention when incorporated into xerographic imaging and printing systems are insensitive to humidity conditions and corona ions generated from corona charging devices enabling these members to generate acceptable images of high resolution for an extended number of imaging cycles exceeding, in most instances, more than 100,000 imaging cycles, and approaching over 500,000 imaging cycles. Also, the specific imaging members of the present invention eliminates the high undesirable lateral movement of charges at the interface between the photoconducting layer, and the silicon nitride overcoating reducing band bending thus enabling images with increased resolution and less print deletions. Furthermore, the barrier layers of the present invention prevent broad area injection of minority carriers, and microinjection sites that cause image defect sites such as white spots. Further, the barrier layers of the present invention prevent the build up of residual potentials. Additionally, the barrier layers of the present invention act as an acceptable adhesive layer.
Electrostatographic imaging, and particularly xerographic imaging processes, are well known and are extensively described in the prior art. In these processes, generally a photoresponsive or photoconductor material is selected for forming the latent electrostatic image thereon. This photoreceptor is generally comprised of a conductive substrate containing on its surface a layer of photoconductive material, and in many instances a thin barrier layer is situated between the substrate and the photoconductive layer to prevent charge injection from the substrate, which injection would lower the charge acceptance and adversely effect the quality of the resulting image. Examples of known useful photoconductive materials include amorphous selenium, alloys of selenium such as selenium-tellurium, selenium-arsenic, and the like. Additionally, there can be selected as the photoresponsive imaging member various organic photoconductive materials including, for example, complexes of trinitrofluorenone and polyvinylcarbazole. Moreover, recently there has been disclosed multilayered organic photoresponsive devices comprised of an aryl amine hole transporting molecule dispersed in an inactive resinous binder and a photogenerating layer, reference U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference. Examples of charge transport layers disclosed in this patent include various diamines, while examples of photogenerating layers include trigonal selenium, metal and metal-free phthalocyanines, vanadyl phthalocyanines, squaraine compositions, and other similar substances.
Additionally, amorphous silicon photoconductors are known, thus for example there is disclosed in U.S. Pat. No. 4,265,991 an electrophotographic photosensitive member with a thickness of 5 to 80 microns comprised of a substrate and photoconductive overlayer of amorphous silicon containing 10 to 40 atomic percent of hydrogen. Further, there are described in this patent several processes for preparing amorphous silicon. In one process embodiment, there is prepared an electrophotographic sensitive member by heating the member in a chamber to a temperature of from 50.degree. C. to 350.degree. C., introducing a silicon and hydrogen containing gas into the vacuum chamber causing an electrical discharge by electric energy to ionize the gas in the space of the chamber in which a silicon compound is present, followed by depositing amorphous silicon on an electrophotographic substrate at a rate of 0.5 to 100 Angstroms per second thereby resulting in a hydrogenated amorphous silicon photoconductive layer of a predetermined thickness. While the amorphous silicon device described in this patent is photosensitive, after a minimum number of imaging cycles, less than about 10,000 for example, unacceptable low quality images of poor resolution with many deletions result. With further cycling, that is subsequent to 10,000 imaging cycles, the image quality continues to deteriorate often until images are partially deleted. Accordingly, while the amorphous silicon photoresponsive device of the '991 patent is useful, its selection as a commercial device which can be used functionally for a number of imaging cycles is not readily achievable.
There are also disclosed in U.S. Pat. No. 4,634,647, the disclosure of which is totally incorporated herein by reference, imaging members comprised of compensated amorphous silicon compositions, wherein there are simultaneously present in the amorphous silicon dopant materials of boron and phosphorus. More specifically, there is disclosed in the copending application a photoresponsive device comprised of a supporting substrate, and an amorphous silicon composition containing from about 25 parts per million by weight to about 1 weight percent of boron compensated with from about 25 parts per million by weight to about 1 weight percent of phosphorus. These members may also contain a top overcoating layer of silicon nitride, silicon carbide or amorphous carbon with stoichiometric amounts of silicon and nitrogen, or silicon and carbon.
Moreover, disclosed in copending application Ser. No. 548,117, now U.S. Pat. No. 4,544,617, the disclosure of which is totally incorporated herein by reference, is an imaging member comprised of a supporting substrate, a photoconducting layer comprised of uncompensated or undoped amorphous silicon, or amorphous silicon slightly doped with p or n type dopants such as boron or phosphorus, a thin trapping layer comprised of amorphous silicon which is heavily doped with p or n type dopants such as boron or phosphorus, and a top overcoating layer of stoichiometric silicon nitride, silicon carbide, or amorphous carbon; and wherein the top overcoating layer can be optionally rendered partially conductive.
The use of specific barrier layers in amorphous silicon imaging members is disclosed, for example, in U.S. Pat. No. 4,359,512. More specifically, there are disclosed in this patent hydrogenated amorphous silicon imaging members with barrier layers of amorphous silicon doped with boron, or other similar substances.
Also, U.S. Pat. No. 4,394,426 discloses hydrogenated amorphous silicon members with barrier layers of undoped silicon nitride; U.S. Pat. Nos. 4,452,874 and 4,452,875 disclose hydrogenated amorphous silicon members with two layers between the photoconductive bulk layer and substrate. It is indicated in these patents that the first adhesive layer of undoped silicon nitride is followed by a barrier layer of amorphous silicon doped with boron. Further, these patents describe three layers situated between the bulk photoconductive layer and the substrate, that is an adhesive layer of undoped silicon nitride, a barrier layer of boron doped silicon, and an adhesive layer of undoped silicon nitride. Presumably the first adhesive layer improves adhesion between the substrate and the second barrier layer, and the third layer improves adhesion between the barrier layer and the bulk. Some disadvantages associated with the aforementioned members are: (1) although the high concentration boron doped barrier layer functions as a barrier against minority carrier injection, it does not possess adhesive characteristics, thus with such a barrier adhesive failure in amorphous silicon imaging members is frequently encountered; and (2) undoped silicon nitride is not an acceptable barrier when the nitride is nonstoichiometric and contains excess silicon. Near stoichiometric silicon nitride although an acceptable barrier against electron injection is highly resistive, and therefore develops a residual potential. Further, although the three layer SiN.sub.x --SiB+--SiN.sub.x structure satisfies the adhesive as well as the barrier needs, the process for the preparation thereof involves changing gases several times to obtain the three layer structure.
Further, there is disclosed in the prior art amorphous silicon photoreceptor imaging members containing, for example, stoichiometric silicon nitride overcoatings; however, these members in some instances generate prints of low resolution as a result of the band bending phenomena. Additionally, with the aforementioned silicon nitride overcoatings, the resolution loss can in many instances be extreme thereby preventing, for example, any image formation whatsoever.
Additionally, described in U.S. Pat. No. 4,613,556, entitled Heterogeneous Electrophotographic Imaging Members of Amorphous Silicon, the disclosure of which is totally incorporated herein by reference, are imaging members comprised of hydrogenated amorphous silicon photogenerating compositions, and a charge transporting layer of plasma deposited silicon oxide.
Other representative prior art disclosing amorphous silicon imaging members, including those with overcoatings, are U.S. Pat. Nos. 4,460,669; 4,465,750; 4,394,426; 4,394,425; 4,409,308; 4,414,319; 4,443,529; 4,452,874; 4,452,875; 4,483,911; 4,359,512; 4,403,026; 4,416,962; 4,423,133; 4,460,670; 4,461,820; 4,484,809; and 4,490,453. Additionally, patents that may be of background interest with respect to amorphous silicon photoreceptor members include, for example, U.S. Pat. Nos. 4,359,512; 4,377,628; 4,420,546; 4,471,042; 4,477,549; 4,486,521; and 4,490,454.
Further, additional representative prior art patents that disclose amorphous silicon imaging members include, for example, U.S. Pat. No. 4,357,179 directed to methods for preparing imaging members containing high density amorphous silicon or germanium; U.S. Pat. No. 4,237,501 which discloses a method for preparing hydrogenated amorphous silicon wherein ammonia is introduced into a reaction chamber; U.S. Pat. Nos. 4,359,514; 4,404,076; 4,403,026; 4,397,933; 4,423,133; 4,461,819; 4,237,151; 4,356,246; 4,361,638; 4,365,013; 3,160,521; 3,160,522; 3,496,037; 4,394,426; and 3,892,650. Of specific interest are the amorphous silicon photoreceptors illustrated in U.S. Pat. Nos. 4,394,425; 4,394,426 and 4,409,308 wherein overcoatings such as silicon nitride and silicon carbide are selected. Examples of silicon nitride overcoatings include those with a nitrogen content of from about 43 to about 60 atomic percent.
Additionally, processes for depositing large area defect free films of amorphous silicon by the glow discharge of silane gases are described in Chittick et al., the Journal of the Electrochemical Society, Volume 116, Page 77, (1969). Further, the fabrication and optimization of substrate temperatures during amorphous silicon fabrication are illustrated by Walter Spear, the Fifth International Conference on Amorphous and Liquid Semiconductors presented at Garmisch Partenkirchen, West Germany in 1963. Other silicon fabrication processes are described in the Journal of Noncrystalline Solids, Volumes 8 to 10, Page 727, (1972), and the Journal of Noncrystalline Solids, Volume 13, Page 55, (1973).
While the above described imaging members, particularly those disclosed in the copending applications, are suitable for their intended purposes, there continues to be a need for improved imaging members comprised of amorphous silicon. Additionally, there is a need for amorphous silicon imaging members that possess desirable high charge acceptance and low charge loss in the dark. Furthermore, there continues to be a need for improved amorphous silicon imaging members with new barrier layers, and overcoating layers of nonstoichiometric silicon nitrides enabling the substantial elimination of the undesirable lateral motion of charge, and thereby allowing for the generation of images of increased resolution when compared to amorphous silicon imaging members with stoichiometric overcoatings of silicon nitride. Additionally, there continues to be a need for improved layered imaging members of amorphous silicon which are humidity insensitive, and are not adversely effected by electrical consequences resulting from scratching and abrasion. There is also a need for amorphous silicon imaging members which can be selected for use in repetitive imaging and printing systems. Furthermore, there is a need for amorphous silicon imaging members which have the property of low surface potential decay rates in the dark, and yet are photosensitive in the visible and near visible wavelength range. Further, there is a need for hydrogenated amorphous silicon imaging members with barrier layers that satisfy the adhesive as well as barrier requirements without having to formulate multilayer structures.